1. Field of the Invention
The invention relates to the field of methods and apparatuses for preventing minority carrier injection, such as caused by forward biased junctions in semiconductor memories.
2. Prior Art
In complementary metal-oxide-semiconductor (CMOS) memories, particularly where an array is formed in a well of a first conductivity type and the well is disposed in a substrate of a second conductivity type, there is always the possibility that a junction can become forward biased. For instance, where a five volt potential is applied to a p-type region in an n-type well where the well is also biased to five volts, power supply noise along with the distributed resistance and capacitance of the well can cause the p-type region to become forward biased. This injects minority carriers into the n-well which results in problems. In the case of dynamic RAMs, these minority carriers can be collected by a memory cell, altering the stored binary state. This problem will be described in more detail in conjunction with FIG. 1.
For the most part, the prior art has avoided this problem by using back biasing. Often, an on-chip generator (charge pumping circuit) produces a potential higher than or lower than the power supply potential to permit back biasing of wells, substrates, or the like. These generators consume relatively large amounts of power, particularly when compared to the standby power required by a CMOS dynamic memory. Thus, while back biasing prevents the undesirable generation of minority carriers, it has the disadvantage of consuming power.
In other dynamic memories, where back biasing is not required, more complex processing is used. Arrays are not fabricated in the more simply formed wells. Rather, epitaxial layers are employed to allow low resistance paths to the substrate.
As will be seen, the present invention provides a relatively easy to fabricate apparatus for preventing the generation of the minority carriers within a well. The invention is particularly useful where dynamic memory cells are formed in a well.